Wideband communications devices typically operate with a wideband complex signal, and can interfere with narrowband communications devices. Interference with narrowband communications devices may cause a loss of critical communication links. For wideband and narrowband communication devices to coexist, the wideband complex signal being transmitted may be filtered to generate notches overlapping the operating frequencies of the narrowband communication devices.
An example wideband communications device is a satellite communications system known as MOOS (Mobile User Objective System). The MOOS satellite communications system operates in the UHF band. The MUOS system is designed to reduce the impact on nearby narrowband communications devices by using an adaptive notch-on-transmit filter.
There are several approaches for providing an adaptive notch-on-transmit filter for notching the MUOS waveform. Each approach uses a different filter bank structure that affects processor loading and memory requirements. For example, a paper titled “Filter banks For Adaptive Transmit Filtering” by Chad Spooner discloses notch-on-transmit algorithms that focus on a DFT filter bank and on a modified DFT (mDFT) filter bank.
The DFT filter bank is applied to the waveform in a sliding block manner. In the mDFT, the input signal is simultaneously applied to each of the sub-band branches of the filter bank. Analysis filters are immediately applied to the input data. Each analysis filter is a frequency-shifted version of a real-valued low pass filter. The outputs of the analysis filters are decimated, and the real and imagery parts of the result are alternately taken over time. The parallel operations result in real and imagery components for each input block of complex numbers, just as in the DFT filter bank. The difference between the mDFT and the DFT filter banks is that the exact sub-band filtering characteristics are under a designer's direct control in the mDFT filter bank. Even in view of the DFT filter bank and the modified DFT (mDFT) filter bank, processor loading and memory requirements are not significantly reduced.
A paper titled “MUOS Spectrum Notching Effect On Handheld Terminal Uplink Performance” by Kumm et al. discusses the effect of the peak-to-average power ratio (PAPR) of the spectrally adaptive waveform in terms of performance. The PAPR increases with an increasing notching bandwidth in the spectrally adaptive waveform as compared to an un-notched waveform. With spectrally adaptive notching, the dynamic range of the communications device transmitting the waveform is “squeezed” by the need to preserve PAPR while meeting a maximum power limit. The paper concludes that there is no straightforward way to reduce PAPR into the power amplifier of the communications device transmitting the waveform so as to boost output power of the required notch depth and out-of-band requirements. In addition, the paper fails to address reducing processor loading and memory requirements when generating the notching in the spectrally adaptive waveform.
In addition, wideband communications devices receiving a wideband complex signal are also susceptible to narrowband interference. Narrowband interference may result from the presence of background UHF interference, legacy interferes and multiple access interference. Narrowband interference affects the signal-to-noise ratio (SNR) of the received wideband complex signal. A paper titled “MUOS U2B Interference Mitigation Analysis” by Bahr et al. discloses how spectral whitening applied at the receiver reduces interference by reducing interference power at the demodulator. In the receiver, the received wideband complex signal including an undesired narrowband interference component therein is applied to a filter comprising a mDFT filter bank. The mDFT filter bank implements the spectral whitening while also notching the undesired narrowband interference component. While the filter comprising the mDFT filter bank is effective at suppressing the undesired narrowband interference component, the use of a mDFT filter still places a demand on processor loading requirements.